This invention relates generally to a gas turbine engine, and, more specifically, to a combustion system for a gas turbine engine.
In at least some known gas turbine engine combustion systems, to facilitate reducing the possibility of lean blow-out, or a blow out caused when the environment has a low fuel to air ratio, diffusion fuel is used to start turbine operation. Depending on the turbine, diffusion fuel injected through fuel nozzles may become concentrated downstream from the fuel injection nozzles. The increased concentration of diffusion fuel may undesirably increase a fuel rich fuel/air ratio downstream from the fuel injection nozzles such that the fuel/air ratio is increases beyond the upper design limit. Such a fuel rich environment may exceed a rich blow out (RBO) boundary causing the diffusion fuel flame to blow out. More specifically, most known rich blow outs occur at about 80% of turbine speed during turbine start up.
Some known combustion systems compensate for the fuel rich environment by reducing the flow of diffusion fuel and injecting a fuel premixed with air before the turbine obtains full operating speed. A turbine start-up that injects premixed fuel before the turbine reaches full speed may be referred to, for example, as a “lean-lean start.” However, because the premix fuel flame is more unstable than diffusion fuel flames, to facilitate stabilizing the flame more fuel must be supplied to a premix fuel flame than to a diffusion fuel flame. For example, in some known systems, approximately 50% or more of the total fuel injected into the combustor is premix fuel injected through one of the plurality of nozzles within the combustor.
In at least some known combustors, a lean-lean start may increase the local liner wall temperature near the premix fuel flame. Such an increase in temperature generally occurs because of the disproportionate amount of premixed fuel supplied to one of the fuel nozzles, as compared to the amount of fuel supplied to other nozzles within the combustor. Moreover, such an increase in temperature may prematurely wear the combustor hardware surrounding the flame, such as, for example, the combustor liner and/or transition pieces. As a result, such combustor hardware may be replaced more frequently than if the start-up combustion temperatures were maintained at a lower temperature. To compensate for higher temperatures, some known combustors include components that are more resistant to thermal wear. Such components may add cost and/or weight to the engine compared to engines having combustors that do not include thermally resistant components.